1. Field of the Invention
The present invention relates generally to the transmission and synchronization of asynchronous digitally encoded data between a central processor and a plurality of terminals or other computer and associated peripheral equipment for utilizing or operating with such transmitted data. More specifically, the invention relates to a system for extending the allowable transmission line length between a computer and its associated terminal devices beyond the length possible with systems of the prior art.
When digitally encoded data, in whatever format, such as phase encoded, sometimes referred to as Manchester encoding, NRZ, or other known code is transmitted over a transmission line, the phase error, which is sometimes referred to as "bit-shift" or peak shift, between the data transitions and the system clock increases with increasing transmission line distance until the phase shift error results in loss of synchronization between clock and data, and errors in the transmitted data when decoded. The present invention relates to a technique for retiming the data at some point or points along the transmission line with a newly generated retiming clock gated on by the recognition of the incoming data, and retransmitting the retimed data along the transmission line, thereby extending the transmission line distance possible between a central processor and its terminals by substantially reducing the phase shift errors which would otherwise occur in transmission.
An exemplary application for such a retiming system is a central processor to which data is coupled from a plurality of electronic cash registers, as, for example in a department store.
2. Description of the Prior Art
Many communications systems are known in the prior art for transmitting digital data serially, in parallel, synchronously and asynchronously between a computer and its associated peripheral equipment. While such data may usually be synchronously transmitted from the computer to the peripheral equipment, data from the peripherals, in multi-byte format, is generally asynchronously transmitted to the computer, since the time of origin of such data is usually randon and intermittent. The present invention relates to an improved data transmission system for retiming and retransmitting such asynchronous data.
A typical communication system of the prior art illustrative of synchronous data communication between a central computer and its data terminals is disclosed by U.S. Pat. No. 3,676,846. A plurality of repeaters inserted in a transmission line for transmitting pulsed information is disclosed by U.S. Pat. No. 3,072,744. Another repeater technique for a bidirectional communication system is disclosed by U.S. Pat. No. 3,040,130. A serial loop data transmission system is disclosed by U.S. Pat. No. 3,633,166. A multi-clock timing control for a multiprocessor system is disclosed by U.S. Pat. No. 3,715,729. None of the above-identified prior art patents utilize the present retiming technique of retiming asynchronous data character by character by switching from data self-clocking to a retiming clock as each character is detected to enable character synchronous transmission and transmission line extension, which constitutes an improvement over such prior art techniques.